fix help message of test_bench.py (#1021)
Summary:
README.md was edited by https://github.com/pytorch/benchmark/issues/133 but help message of test_bench.py is still saying `python test.py --setup_only`.
Fixed it.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1021
Reviewed By: george-qi
Differential Revision: D37760218
Pulled By: xuzhao9
fbshipit-source-id: 9e175ea68e7a4aa46509a2462748ad0c024886ab
